Most viruses that use email as a vehicle are aimed to take advantage with vulnerabilities with Windows, so it is unlikely that the common spam / virus email will be able to comprimise your iPhone.
I also use gmail, so most of the email I get has the spam / viruses filtered out by default, of which has never let me down.
But let's not forget common sence, if you don't know the sender, don't open the email.
ILMV